A wind turbine converts wind power into electrical energy through a generator driven by the blades of the wind turbine under windy conditions. In some wind turbines, the generator comprises a stator and a rotor both having multiphase windings, such as a double-fed induction generator, a cascade-regulated generator, an induction or a synchronic generator, or even a DC generator.
When having a rotor rotating in relation to a stator of a generator, electricity has to be transferred to or from the rotor. Electricity may be transferred by means of a slip ring assembly also called a rotary electrical interface, rotating electrical connector, collector, or swivel or electrical rotary joint, enabling the rotor to receive or return electricity while rotating in relation to the stator. When having a rotor with a multiphase winding, a control system within the wind turbine is able to regulate the frequency, so that the wind turbine produces at a predetermined frequency, e.g. approximately 50 Hz.
In order to control the magnetic field within the generator, the slip ring unit may be provided with a shaft which is connected with an encoder. Due to the fact that the slip ring unit transfers electricity to or from the rotor, the shaft has been electrically insulated so that the encoder is not destroyed by electricity unintentionally transferred from the slip ring unit. The insulation is provided by means of an insulation shaft in plastic housing the first shaft, which insulation shaft is inserted into the encoder. In this way, the encoder can no longer be destroyed by unintentional electricity.
However, in production it has been very difficult to keep the outside diameter of the plastic shaft within the allowable dimension tolerance in relation to the encoder, as a consequence of which the encoder may fail to count the actual number of turns of the rotor.